1. Field of the Invention
The present invention relates generally to a wet etching for forming a metal film pattern, and more particularly, to an improvement in etching for forming a metal film pattern (hereinafter referred to as a taper-etching) having tapered edges.
2. Description of the Background Art
FIG. 1 is a flow chart illustrating a conventional wet etching for forming a metal film pattern. As can be seen from the diagram, a metal film is deposited on a substrate of silicon or glass by, for example, sputtering in step S1. In step S2, the metal film is covered by a photoresist layer including phenol novolac as a main component. In step S3, the photoresist is pre-baked. In step S4, predetermined regions of the pre-baked photoresist layer are exposed to light by known photolithography. In step S5, the exposed photoresist layer is developed to have a predetermined pattern. The photoresist pattern is dried in step S6 and fully cured by post-baking at a fixed temperature for a fixed time in step S7. Then in step S8, the metal film covered by the resist pattern is etched to have a predetermined pattern and by a known wet etching method such as a shower method, a spray method and a paddle method with appropriate etchant.
FIG. 2 schematically shows a section of a thin film transistor (TFT). The TFT is commonly used as a drive transistor in a liquid crystal display apparatus. In the TFT, a gate electrode 2a is formed on an insulator substrate 1 of glass or the like by etching a metal film of chromium or tantalum. The gate electrode 2a is covered with a first insulator layer 3 of SiO.sub.2 or Si.sub.3 N.sub.4. An intrinsic type amorphous silicon layer 4 is deposited on the first insulator layer 3. The intrinsic type amorphous silicon layer 4 is covered with a second insulator layer 5 of Si.sub.3 N.sub.4. An n.sup.+ type amorphous silicon layer 6 is deposited on the second insulator layer 5. Source/drain regions of the n type amorphous silicon layer 6 are in contact with the intrinsic type amorphous silicon layer 4 through holes formed in the second insulator layer 5. Source/drain electrodes 7 of aluminum or the like are formed on the source/drain region of the n type amorphous silicon layer 6. The source/drain electrodes 7 are covered with a protective insulator film of SiO.sub.2 or the like.
In such TFT as shown in FIG. 2, edges of the second gate electrode 2a are desirably tapered in order to obtain an excellent coverage of upper partial layers in the vicinity of the edge and avoid undesired electric field concentration in the vicinity thereof. The reason is that an improved coverage attributes to prevent disconnection of a lead line, thereby improving a yield rate and that the attenuation of the electric field concentration increases a breakdown voltage of the TFT.
FIGS. 3A, 3B and 3C are sectional views showing the taper etching disclosed in Japanese Patent Laying-Open No. 64-86524. In FIG. 3A, a chromium film 2 is formed on a substrate of silicon or the like by plating or vacuum evaporation, for example. A pattern 10 of photoresist OFPR-77E (product of Tokyo Ohka Corporation) with phenol novolac as a main component is formed on the chromium film 2 by known photolithography.
Thereafter, the chromium film 2 is etched with etchant including ammonium cerium (IV) nitrate of 19 g, nitric acid of 13 cc and water of 87 cc. At this time, the nitric acid included in the etchant starts peeling the edges of the resist layer 10 off from the metal film 2 and the etchant dissolves the chromium film 2 at the same time, as shown in FIG. 3B. As a result, the edges of the patterned chromium film 2a are tapered to have an inclination angle .theta. as shown in FIG. 3C.
The taper angle .theta. can be controlled by adjusting the temperature and the concentration of the nitric acid of the etchant as shown in FIG. 4. In the graph of FIG. 4, the abscissa represents the concentration of nitric acid (mol/l) and the ordinate represents a taper angle .theta. (deg). The curves A, B, C and D respectively show the etching with the etchant temperatures of 22.degree. C., 32.degree. C., 42.degree. C. and 52.degree. C.
Such taper-etching according to the prior art as described above requires the concentration of nitric acid and the temperature of the etchant to be controlled when the thickness of the metal film 2 and the taper angle .theta. are changed. In addition, it is difficult to control the taper angle of 20.degree. or less with precision, as can be seen from the graph of FIG. 4. Furthermore, the surface of the metal film 2 is liable to become nonhomogeneous with the passage of time after the deposition thereof, because the surface is partially oxidized or adsorbs moisture. The nonhomogeneous surface of the metal film 2 results in variations of the taper angle .theta. depending on the location on the substrate.